Have you ever wondered what conditions are required for a combined heat pump and thermal energy storage system to save energy or reduce peak demand?

In this system, the TES acts as a third temperature body in addition to the application of interest (e.g., indoor space temperature) and the ambient temperature (e.g., outside air). This three nodal system allows use to take advantage of naturally occurring diurnal ambient temperature fluctuations to achieve energy savings.

This paper explores the operating 24 scenarios of such a HP-TES system using thermodynamic first principles and validated and trusted numerical HP models to show the thermodynamic limits of energy savings and peak demand reduction. The choice of TES temperature relative to the application temperature, whether the system operates in a heating or cooling mode, how the on-peak period is defined, and the uncontrollable and elusive ambient temperature fluctuations largely dictates the extent to which actual energy savings and demand reduction are seen.

In the thermodynamic limit we show that choosing a TES temperature near to the application temperature might see the highest potential for energy savings and demand reduction. 

This figure shows the energy consumption for space heating for a conventional HP system and a HP-TES system.  The inclusion of TES inverts the energy demand of the building.  This has implications for the  grid generation and associated emissions.

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Abstract

Thermal energy storage (TES) decouples heat generation from use, providing a crucial tool to mitigate fluctuating thermal loads.   TES systems may often contain a phase change material (PCM) which stores heat isothermally through the enthalpy of phase change, and thus the TES as a whole operates at a near constant temperature.  Integrated into a heat pump (HP) or heat transfer fluid (HTF) circulation loop, the TES will behave as an isothermal heat exchanger (HX), absorbing or releasing heat into the HTF.  For near-ambient TES (-15-85°C), solid-liquid PCMs provide the highest energy storage density.  However, the PCMs available in this temperature range often have low thermal conductivity that hinder the power capacity of a TES.  As such, modifications such as metallic extended surfaces or fins are made to the TES to increase its power capacity.  This work instead uses an enhanced PCM composite material in a simple and scalable shell-and-tube design.  In this work, two prototype-scale TES units are demonstrated: 1) a benchtop scale unit as a proof-of-concept with targeted specifications 50 W, 100 Wh, and 2) an intermediate scale with target specifications 200 W, 0.8 kWh.  The PCM composite is based on sodium sulfate decahydrate (SSD) and high thermal conductivity expanded graphite (EG).  Both TES units containing this material met or exceeded design performance.  The design, production method, and performance results are discussed.

The two figures show (A) the normalized effective thermal effusivity which is a metric representing the thermal responsiveness of the phase change and (B) the normalized energy storage density.  As graphite is added to the PCM composite, its thermal conductivity does increase, but its density decreases.  The normalized effective thermal effusivity captures this tradeoff and shows there is a loss in thermal responsiveness at high graphite fractions.  Thus, despite the highest thermal conductivity, these composites have no better thermal response than the pure PCM.  Furthermore, figure (B) shows that adding graphite will necessarily decrease the energy storage capability of the PCM composite which effects the system-wide design of such systems.

Script

We are surrounded by batteries.  Our phones, computers, watches, cars, and more all contain devices that store electrical energy.  However, our lives are lived immersed in thermal energy.  We expect our refrigerators, freezers, and air conditioners to be cold.  And our furnaces and hot water to be hot.  We expect our homes and workplaces to be comfortable.  When this doesn’t happen, it can lead to dissatisfaction, agitation, discomfort, but in the worst-case scenario, it can lead to death like we saw in Texas a year ago.

Many of the systems I just listed operate with an instantaneous conversion from electrical energy to thermal energy, and they are dependent on an uninterrupted and secure electric power grid.  My dissertation seeks to lessen this dependence, enabling these systems to operate with more intermittent renewable energy sources without loss of thermal comfort. 

This is done with time travel.  We can air condition our homes by storing the relative coolness of night for tomorrow’s hot summer day.  Or we can store warmth from the midday sun to heat our water in the dead of winter.  Thermal energy storage enables us to capture the Earth’s natural heat and thermal cycling, and use it hours or even days later when we need it.  And by storing this energy as heat, we remove inefficiencies of electrical-to-thermal energy conversion.

In my research I’ve shown through model simulations that implementing thermal energy storage in a building air conditioning and heating system can reduce the total energy consumption from the electric grid while maintaining the same level of indoor thermal comfort.   But perhaps more importantly, these models show that the grid electric energy consumption can be reduced by nearly 60% during the most critical times of the year – including extreme weather events.  This means that during summer heat waves or winter cold snaps, we may not be so reliant on carbon dioxide emitting power plants, but can instead harness the heat already around us to satisfy our thermal comfort requirements.

This technology is transformative and necessary for the complete transition to renewable energy.  Soon, thermal energy storage systems will be as ubiquitous as the electric batteries that surround us.  But they will be invisible, silently strengthening our energy grid in the background without us breaking a sweat. 

This figure shows the stability of sodium sulfate decahydrate (SSD) with dextran sodium sulfate (DSS).  a) shows the initial state of the SSD with 0, 0.5, and 5 wt% DSS.  b) shows the final state after 30 minutes of dwell in the liquid state. c) shows the viscosity of the three samples.  d) shows the energy storage capacity of the three samples after 25 thermal cycles (melt/freeze).

This figure compiles the energy storage material cost ($/kWh) for common phase change materials with 1st order phase change between 0-65C.  The energy storage material cost is the bulk industrial supplier raw material cost ($/kg) divided by the latent heat of phase change (kWh/kg).  The methodology and data labels are available in the full text.

This figure shows the possible energy savings of a thermal energy storage (TES) system for cooling a single family detached home in California.  The choppiness of the graphs show the timing of the air conditioning system.  Significant on-peak hour energy reduction is modeled.  The full analysis and cost information is available in the full text.

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Abstract

Phase change materials (PCMs) are valuable for their ability to store heat nearly isothermally around their phase transition temperature. PCMs are at the core of latent heat thermal energy storage (LHTES) systems, which provide the ability to buffer high thermal loads or decouple the time when heating or cooling is needed from when it is produced. Thermal charging and discharging of LHTES systems often employ a constant temperature source, and the rate at which heat can be exchanged is dependent on the thermophysical properties of the PCM. For graphite composite PCMs, the high thermal conductivity of the graphite enables an increased heat transfer rate through the material, but its presence displaces PCM which reduces the effective volumetric latent heat of the composite relative to the pure PCM. This results in a tradeoff between thermal power and volumetric energy storage capacity. The thermal charging rate is the average thermal energy stored in the material for some elapsed time. In this study, composite PCMs of compressed expanded natural graphite (CENG) and n-Octadecane are studied. Samples with varying CENG mass fractions were synthesized and the thermal charging rate was measured under a constant temperature boundary condition. For evaluation of the expanded graphite-PCM composite, one boundary of the material was exposed to a 50°C constant temperature plate, above the 27.5°C melting temperature of the PCM. The melting front progression [mm-s-1 ] and the thermal charging rate [W-cm-2 ] of the PCM were determined, and the results were compared with analytical predictions for the 1-D semi-infinite phase change. For CENG addition up to 5.75% mass fraction, a 450% thermal conductivity increase was observed with a only 5% decrease in volumetric energy density as compared to pure octadecane. The average thermal charging rate was increased by over 430% for the melt to penetrate a depth of 22 mm. The experimental results matched analytical predictions, indicating that higher CENG fractions can be evaluated using analytical approaches. 

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Abstract

The molecular structure and dynamics of water differ considerably at various interfaces. We compare the interfacial water structure–property relationship on three different carbon substrates, namely, amorphous carbon, compressed expanded natural graphite, and pure graphite by utilizing atomistic molecular dynamics simulations. The effect of different substrates on the structural and dynamical properties of water can readily be observed. The density distributions parallel and normal to the substrates show oblate droplet structures. The normal to the substrate water distribution shows a strong hydration layer at the interface that does not vary with substrates. However, the disparity in the structure and dynamics on three different substrates shows that the surface morphologies of the substrates are critical for determining nanoscale water properties. Furthermore, it is observed that the formation of an interfacial water layer or the hydration layer is a direct consequence of both water “confinement” at the nanoscale and “attraction” between water molecules and the carbon substrates.

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Abstract

Salt hydrate-based phase-change materials are considered promising for future heat storage applications in residential heating/cooling systems. Smooth phase transition from the liquid to solid phase and vice versa is essential for effective heat exchanger; however, supercooling in salt hydrates delays the onset of liquid–solid phase transition. We investigate the molecular level mechanism of supercooling in sodium sulfate decahydrate (SSD). SSD is a complex salt hydrate whose properties are governed by electrostatic forces that include pure Coulombic interactions as well as hydrogen bonds. Experimentally, we examine the importance of a nucleator in reducing supercooling temperatures. We investigated the effect of various mass concentrations of a borax nucleator on a decrease of supercooling temperatures. Molecular dynamics simulation techniques are used to obtain a basic understanding of supercooling in SSD. We observe that by introducing borax as a nucleator, there is a decrease in the supercooling temperature before nucleation. Our molecular dynamics simulations show that long-range electrostatics between sodium and sulfate ion pairs and that with polar water molecules is responsible for delayed nucleation in SSD that results in supercooling, and also, dynamics of charged molecules slows down. The lack of crystallization leads to amorphous structures in supercooled SSD.

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Abstract

Salt hydrates can be used as phase change materials for thermal energy storage. Critical technical challenges for their widespread deployment include poor cycling stability, large supercooling, and low thermal conductivity. In this work, numerous enhancement techniques are reviewed. Related to stability, numerous existing methods to characterize the number of thermal cycles are summarized. Following this, 11 techniques to mitigate phase separation (plus 9 studies on macro-, micro-, and nano-encapsulation types) are reviewed. For supercooling, 38 nucleator-salt hydrate combinations to minimize subcooling are reviewed. The empirically observed effect of isomorphism in minimizing subcooling is explored and discussed, with cross-study trends depicted for 38 nucleators across 9 salt hydrates. In addition, several studies reporting combined effects on stability and supercooling are presented. Related to thermal conductivity, 32 combinations of conductivity enhancement material and salt hydrate are reviewed. For those using graphite, the dependence of conductivity enhancement on graphite mass fraction is shown across numerous studies for different graphite types. The performance and stability of calcium chloride hexahydrate, sodium sulfate decahydrate, and sodium acetate were explored and are discussed in-depth. Finally, the status of enhancement to salt hydrates is summarized, and remaining challenges are identified.

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Abstract

Phase change materials (PCMs) with desirable phase change temperatures can be used to provide a constant temperature source or sink for diverse applications. As such, incorporating PCMs into building materials, equipment, or appliances can shift and/or reduce the energy load. The motivation of this work is to identify low-cost inorganic salt hydrate PCMs that can complement current building systems and designs. Two key challenges to incorporating PCMs into building materials are 1) maintaining desirable thermal properties at large scales, and 2) developing cost-effective systems that are easily incorporated into existing structures and systems. In this work, we present an analysis of inorganic salt hydrates with phase change temperatures in the range of 5-60 C, targeted towards both space heating and cooling. The properties of the salt hydrates are compared with common waxes over the same temperature range. The results showed that salt hydrate systems such as sodium thiosulfate pentahydrate, NaS2O3· 5H2O, has a latent heat as high as 201 kJ/kg at a phase change temperature of 48 C which is comparable to some paraffin waxes (213 kJ/kg at 52.5 C). At a density of 1.73 g/cm3, sodium thiosulfate pentahydrate has an energy density of 347 J/cm3 (paraffin waxes, 170 J/cm3). Moreover, it was found that salt hydrates are generally less costly per unit energy in contrast to common waxes with typical salt hydrate costs in the range of 0.001-0.01$/kJ. This analysis shows the potential of salt hydrate PCMs for developing low-cost heating and cooling thermal energy storage systems to meet a range of applications. 

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Abstract

Heat is fundamental to power generation and many industrial processes, and is most useful at high temperatures because it can be converted more efficiently to other types of energy. However, efficient transportation, storage and conversion of heat at extreme temperatures (more than about 1,300 kelvin) is impractical for many applications. Liquid metals can be very effective media for transferring heat at high temperatures, but liquid-metal pumping has been limited by the corrosion of metal infrastructures. Here we demonstrate a ceramic, mechanical pump that can be used to continuously circulate liquid tin at temperatures of around 1,473–1,673 kelvin. Our approach to liquid-metal pumping is enabled by the use of ceramics for the mechanical and sealing components, but owing to the brittle nature of ceramics their use requires careful engineering. Our set-up enables effective heat transfer using a liquid at previously unattainable temperatures, and could be used for thermal storage and transport, electric power production, and chemical or materials processing.